TSLP-activated DCs polarize naive CD4 T cells into IL-21–secreting cells. Untreated DCs, treated with TSLP (TSLP-DC) or LPS (LPS-DC) were cultured with naive CD4 T cells for 6 d. (A) CBA (IL-4, IL-13, IFN-γ, and IL-17A) and ELISA (IL-21) assays after 24 h of restimulation with anti CD3/CD28 beads. Th0, naive T cells cultured for 6 d with anti-CD3/CD28; Th17, Th0 plus Th17 polarizing cytokines (IL1β, IL-23, TGF-β, and IL-6). Data are mean ± SEM from nine independent experiments. (B) Intracellular FACS staining for IL-21, IFN-γ, TNF, and IL-4 for one representative donor. Gate is on activated DAPI⁻ CD4 T cells. (C) Quantification of data as in B. Data are mean ± SEM from six independent experiments. (D) Distribution of IL-21⁺ cells (red square) polarized by TSLP-DC coproducing IL-4, TNF, and IFN-γ. Filled histogram, isotype control; black line, IL-21 staining. Mean of six independent experiments. Single IL-21 producers (16%) are not plotted. *, P < 0.05; **, P < 0.01; ***, P < 0.001, by Wilcoxon or Student’s t test.